Re: [Boost-Users] mpl::if_ or mpl::apply_if?

From: Terje Slettebų (tslettebo_at_[hidden])
Date: 2003-02-24 15:39:43

>From: "Markus Werle" <yg-boost-users_at_[hidden]>

> I am trying to migrate some parts of my code from
> loki to boost::mpl.
>
> Q1: which is the best way to perform nested if-then-else?
>
> Assume given "values":
> bool B1, bool B2;
> class ff, class tf, class ft, class tt;
>
> I replaced
>
> -----------------------------------------------------------
>   typedef typename
>   Loki::Select<B1,
>                typename Loki::Select<B2, tt, tf>::Result,
>                typename Loki::Select<B2, ft, ff>::Result
>                >::Result type;
> -----------------------------------------------------------
>
> with
>
>   typedef typename
>   apply_if
>   <
>    bool_c<B1>,
>
>    apply_if
>    <
>     bool_c<B2>,
>     identity<tt>,
>     identity<tf>
>    >,
>
>    apply_if
>    <
>     bool_c<B2>,
>     identity<ft>,
>     identity<ff>
>    >
>   >::type type;
>
> Q2: Is apply_if always to be preferred over if_?

mpl::if_ is the same as Loki::Select (actually, mpl::if_c is). mpl::apply_if
does a "::type" on the result. Thus, the following two are equivalent:

mpl::if_<...>::type::type
mpl::apply_if<...>::type

This is useful for delaying evaluation. For example:

// Add reference if "flag" is true

typedef mpl::if_c<flag,
  boost::add_reference<T>::type,
  T
>::type new_type;

If T is void, the above will give an error (reference to void), even if
"flag" is false. This is because "add_reference<T>::type" instantiates the
template, regardless of the outcome of the if_c. However, if you instead do
it like this:

typedef mpl::if_c<flag,
  boost::add_reference<T>,
  mpl::identity<T>
>::type::type new_type;

then there won't be a problem, because add_reference<T> doesn't get
instantiated in this case. The above may be written as:

typedef mpl::apply_if_c<flag,
  boost::add_reference<T>,
  mpl::identity<T>
>::type new_type;

Thus, this is an important idiom: Preventing eager evaluation of the
arguments. apply_if just gives a handy way of doing it. If you don't need
this, then if_ can be used just as well.

> Q3: (OT) Is there a mpl-equivalent to run-time case?

I don't think so. That could be a good addition.

Regards,

Terje